The Marley Mid-East War was a conflict between the nations of Marley and the Mid-East Allied Forces on the continental mainland following the failure of the Paradis Island Operation. The war resulted in the surrender of the Mid-East Allied Forces, and Marley's need to acquire the Founding Titan was more fully realized after witnessing the destructive power of anti-Titan artillery.
Prelude: In Marley's operation to take the Founding Titan from the royal family of Paradis Island, their mission was met with failure. Not only were the Warriors unsuccessful in taking the Founding Titan, but the Female Titan and the Colossal Titan were lost during the assault on Stohess District and the battle of Shiganshina District, respectively. With the loss of two inheritors of the Power of the Titans, Marley's military capabilities were greatly reduced.
The war: While the specific events of the war between Marley and the Allies are unknown, Marley successfully gained the upper hand over the Allies over the course of four years. A significant portion of the combat took place on the sea, where Marley slowly pushed back the Allies' fleet and gained control of the waters. However, the naval battles were evidently poorly fought, as Marley had lost over half of its navy to the more technologically advanced Allied ships during these four years.
Naval battles and early conflicts: While the specific events of the war between Marley and the Allies are unknown, Marley successfully gained the upper hand over the Allies over the course of four years. A significant portion of the combat took place on the sea, where Marley slowly pushed back the Allies' fleet and gained control of the waters. However, the naval battles were evidently poorly fought, as Marley had lost over half of its navy to the more technologically advanced Allied ships during these four years.
Battle of Fort Slava: In the year 854, Marley had succeeded in pushing the Allied fleet to the naval station near Fort Slava on the coast of the mainland. At the request of the navy, ground forces were sent to take over the fort before Marley's fleet could advance upon the Allies' fleet stationed in the bay. While the Titan Warriors waited on standby, Commander Magath of Marley's military led a unit of 800 Eldians in a siege against the fort.
Source: Marley Mid-East War (Anime), available under CC BY-SA 3.0.